Tritiated water, or super-heavy water, can be denoted as HTO, and T.sub.2 O, wherein H refers to elemental hydrogen, O refers to elemental oxygen, and T or D refer to elemental hydrogen isotopes having either two or one additional neutron(s) respectively in a hydrogen nucleus. Of course, T.sub.2 O is unstable in H.sub.2 O and generally forms HTO. Tritiated water occurs naturally in very low concentrations. In addition, light water nuclear power plants and nuclear material production facilities produce tritiated water when light water is exposed to ionizing radiation. Light water exposure may be for the purpose of making tritiated water or for heat transfer, cleaning, or any number of applications. An important source of tritiated water is from cooling water of nuclear reactors. Another source is cleanup effluent. It is necessary to process cooling water or cleanup effluent to minimize release of tritiated water into the environment.
Over time, tritiated water has been released from nuclear facilities into the environment at various sites throughout the world and throughout the United States. The released tritiated water makes its way into aquifers or surface waters, thereby making them unfit for drinking or recreation. Although tritiated water at particular sites is in concentrations in excess of safe drinking water standards, its concentrations are below levels considered economical for recovery. For example, at the Hanford site in Washington State, tritium has been measured in the groundwater in concentrations ranging from less than 20,000 pCi/L to over 2,000,000 pCi/L.
Present methods of tritiated water separation include liquid-phase/catalytic exchange, Girdler-Sulfide process, ammonium sequestering, and distillation. All of these processes require significant energy consumption and much capital equipment with attendant high capital and operating costs.
Prior to the present invention, there was no known membrane or membrane process for separating tritiated water from light water. Laboratory work by A. G. Chmeilewski et al. as reported in Journal of Membrane Science, 55, 257 (1989) demonstrated use of cellulose acetate membranes in a pervaporation process separating deuterium oxide from light water. However, Chmeilewski et al. concluded that "the increase in alpha [the separation factor] has not been large enough to warrant further study, at least for purposes of practical application".
Until the advent of the present invention, there was no known membrane method or apparatus for economically separating tritiated water constituents from light water either before or after release of water to the environment.